1. Field of the Invention
The present invention relates to a fixing device to heat and fix an unfixed image such as a toner image carried on a recording member in an image forming apparatus such as electrophotographic copiers, printers, facsimiles and the like.
2. Description of the Related Art
Fixing devices used in image forming apparatuses such as copiers, printers, facsimiles and the like which form print images by electrophotographic methods generally are provided with a rotating heating member such as a heating roller, heating belt or the like to heat and fix an unfixed image on a recording member, and a recording member bearing an unfixed image such as a toner image is passed between a heating member and a rotating pressure member such as pressure roller or pressure belt arranged opposite said heating member, so as to accomplish heat-fixing of the unfixed image under pressure.
Heretofore, rotating heating members heated by radiant heat from a heater such as a halogen lamp heater have been used frequently. In fixing devices using rotating heating members which utilize halogen lamp heaters or the like as a heat source, however, the speed of the temperature rise is quite slow from the time current starts flowing to said heater until the surface of the rotating heating member attains a predetermined constant temperature, thereby making the apparatus difficult to use due to the lengthy preheating time (i.e., the so-called warm-up time) required from the moment the image forming apparatus main power source is turned ON until the rotating heating member attains a predetermined constant temperature. In order to slightly alleviate this disadvantage, fixing devices have been realized which have thinner and smaller diameter rotating heating members, and so require less heat capacity.
An example of reducing the warm-up time by adding a smaller heat capacity by using a thin-layer rotating heating member is disclosed in Japanese Laid-Open Patent Application No. 59-189381, which proposes using a heating roller formed so as to rotate a resistance heating member, which comprises a material that generates heat via a current flow, in conjunction with a core roller, and using an electromagnetic induction heating means including an induction coil arranged in close proximity to a rotating heating member.
Although the thin-layer rotating heating member can be heated rapidly, the surface temperature of such rotating heating member fluctuates greatly due to the small heat capacity even with slight heat transfer from the rotating heating member to other elements, unlike thick-layer rotating heating members. Naturally, this tendency becomes greater as the rotating heating member becomes thinner.
This condition results in a rapid drop in the temperature of the rotating heating member as the rotating heating member is rotated after warm-up when heat is lost to the rotating pressure member which is rotated in contact with said rotating heating member, and when heat is lost to a recording member directly after said recording member is inserted between the rotating heating member and the rotating pressure member.
Inadequate fixing and fixing of irregular strength results when an unfixed image is fused to a recording member with the temperature of the rotating heating member in the aforesaid reduced state.
Of course, although the temperature of the rotating heating member is normally controlled at a predetermined temperature via feedback control by detecting the surface temperature of the rotating heating member using a temperature detecting means, the temperature detecting means (typically a thermistor) used in such temperature control has poor responsiveness and results in temperature dispersion until the temperature detecting means responds. The influence of temperature dispersion is particularly excessive in regulating a constant temperature when accomplishing fixing under heat and pressure using a rotating heating member having a small heat capacity, and makes it difficult to maintain the surface temperature of the rotating heating member at a constant temperature.
Although the temperature dispersion of the rotating heating member is minimized if printing starts after the rotating heating member temperature is adjusted as the rotating heating member is rotated so as to allow adequate heating of the rotating pressure member and peripheral elements, this arrangement controverts the object of shortening the warm-up time by reducing the heat capacity of the rotating heating member, such that when the heat capacity of the rotating heating member is reduced, a temperature drop cannot be avoided when a recording member is inserted even when, for example, printing starts after the rotating pressure roller and peripheral elements have been adequately heated.
In this regard, Japanese Patent No. 61-45831 discloses on Lines 14 through 22 of column 3, Page 2, "The embodiments of the present invention provide that, in the fixing device of FIG. 1, a heating member 4 is forcibly turned ON directly after a slow warm-up of the sensitivity of a temperature detector 3, to forcibly heat a fixing roller 1 until the temperature of the peripheral elements is elevated and the sensitivity of said temperature detector 3 attains a suitable level." In the case of fixing devices using a heating roller having a built-in heater, the image forming apparatus using said fixing device initially turns ON a heater until the heating roller attains a predetermined temperature, and said heater is forcibly turned ON when the heating roller starts to rotate and the fixing device starts operation after the warm-up ends so as to prevent a temperature drop of the heating roller during the initial period of use of the fixing device, and thereby avoid concomitant inadequate fixing.
It is not desirable, however, to have the heater always turned ON by at same power level when forcibly turning ON the heater after warm-up ends. The situations are completely different when again warming up directly after printing several sheets, and when starting warm-up with the pressure roller pressed against the heating roller and the elements peripheral to the fixing device all in low temperature states as when the image forming apparatus has not been used for an extended period.
That is, when the pressure roller and peripheral elements of the fixing device are warm before the start of the heating roller warm-up, as when reheating the heating roller directly after printing several pages, there is only slight heat transfer from the heating roller to the other elements when the heater is forcibly turned ON after the aforesaid warm-up and the heating roller attains an excessively high temperature, thereby causing the unfixed toner to become too molten and transfer onto the heating roller with the likely possibility of causing so-called high-temperature offset. The problem of high-temperature offset becomes more pronounced as the heating roller becomes thinner and the heat capacity becomes smaller.
The present invention provides a fixing device comprising a rotating heating member heated by a heating means, a rotating pressure member which presses against said rotating heating means, a temperature detecting means to detect the temperature of said rotating heating means, and a control means to control the electric power supplied to said heating means based on the temperature detected by said temperature detecting means, said fixing device being of a type which fixes an unfixed toner image on a recording member by having a recording member bearing an unfixed toner image pass between said rotating heating member and said rotating pressure member, and wherein the warm-up time of said rotating heating member can be shortened by using a thin-layer rotating heating member and a heating means capable of rapidly and directly heating said rotating heating means, and wherein temperature fluctuation of the rotating heating member is controlled when a recording member is inserted between said rotating heating member and said rotating pressure member after the rotating heating member has been warmed up so as to produce fixed images of excellent quality. And the present invention provides A heating method comprising steps of warming-up a rotating member, rotating said rotating member before end of said warming-up step, detecting the temperature of said rotating member, and controlling the temperature of said rotating member to maintain a predetermined temperature after said warming-up step, in accordance with the rate of temperature rise during rotating operation of said rotating step.